Presbyopia is a condition where the eye has diminished ability to focus on near objects, a so-called reduction in the ability of the eye to “accommodate” (change the shape of the lens). Usually presbyopia progressively gets worse after a certain age of an individual, typically starting after the age of 40. Young children nearly all have the ability to focus their eyes from infinity down to as close as a distance of 50 mm, after correction for any far or near sight (20 diopters of optical power). However, by the age of 50 the average person can only accommodate their eyes by 2 diopters, a ten to one reduction.
U.S. Pat. No. 3,305,294, issued Feb. 21, 1967 to Alvarez, Ref. [5], which is incorporated herein by reference in its entirety, describes a two-element variable-power spherical lens in which the two elements slide across each other to vary the power. That lens is referred to below as an “Alvarez lens.”
Recently PixelOptics Corp. (www.pixeloptics.com) has developed glasses that can allow a user to switch focus from their far distance prescription to their prescription for reading. This is accomplished using a Liquid Crystal modulator made by the Panasonic Healthcare Company. Devices having continuous focusing ability include: Superfocus (www.superfocus.com) and Adlens (www.adlens.com), which manufacture eyeglasses that employ a flexible membrane lens; Centre for Vision in the Developing World (www.vdwoxford.com) that manufactures glasses that employ an Alvarez lens; and Eyejusters (www.eyejusters.com), who also use an Alvarez lens. In order for such devices to work properly, and to automatically adjust the prescription commensurate with the task, some type of driving signal must be obtained from the user. All of the above mentioned companies are supplying glasses with an adjustment knob, hardly an automatic system. Such an approach is cumbersome, especially when the user has both hands busy, such as in a driving situation. A more recent model from PixelOptics is activated by changes in the output of an accelerometer when the user changes the position of his or her head (head faces downward when the reading glasses prescription is required). However, basing the switch of prescription on moving one's head downward may be appropriate for reading but inappropriate for many other common situations. For example, in the case where someone is walking down a staircase, looking downward, the refocusing of glasses might result in a loss of orientation and potentially incurring an injury. A more reliable way is to monitor the eye parallax, which provides sufficient information to determine the distance to the intended object and therefore of the task at hand.
Distance accommodation in human vision is associated with two mutually connected processes: eye focusing and stereovision. In close distance accommodation, the human eye rotates to target such that the axis of each eye is aimed at the object. The angle between these two axes is the parallax angle. The closer the distance of an object the larger the parallax angle associated with the object. Fortunately, the parallax angle can be monitored and determined by using an eye tracking technique. The data from the eye tracker can provide the required signal to properly control the autofocusing device on the eyewear. There are four main approaches presently used for eye tracking: Electro-OculoGraphy (EOG), scleral contact lens/search, Video-OculoGraphy (VOG), and the video-based combined pupil and corneal reflection technique. See Ref. [1].
The EOG technique relies on the measurement of the skin's electric potential differences, using electrodes placed around both eyes. The scleral technique uses special contact lenses with an embedded coil. These techniques are too cumbersome for the typical user of the proposed autofocusing eyewear.
VOG is based on recording of an image of the corneal limbus, and can potentially be designed as a compact device. Nevertheless this technique requires the mounting of two imaging cameras in front of the eyes, either above or below each limbus (just outside of a person's direct view). The presence of such attachments would be uncomfortable and severely limit the number of uses of the eyewear. In addition, when the eye changes its direction of gaze, the limbus image is severely deformed and sophisticated post processing is required to determine the direction of gaze based on the image. This post processing would result in a delay in the response of the eyewear and also the system would likely be plagued by low accuracy.
A video-based system that combines the techniques of pupil reflection and corneal reflection can be more accurate than any of the previously described approaches. In this technique the eye is illuminated by a collimated near-infrared beam. See Refs. [1, 2]. The positions of bright pupil image and the corneal reflection (first Purkinje point) provide sufficient information to determine the direction of the eye gaze. The first Purkinje point is defined as the virtual primary focus of the front surface of the cornea, treated as a convex mirror, with respect to collimated light incident from a direction straight in front of the head of the subject. The first Purkinje point is typically located at 3.875 mm behind the cornea. The line connecting the Purkinje point and the center of the eye pupil is the actual eye axis, which is in fact the gaze direction. Measuring the mutual pupil center and corneal reflex position is a direct method for measuring the direction of gaze. This approach can be performed very quickly and promises higher accuracy than the other approaches. Unfortunately, contemporary cornea/pupil reflection measurement devices are bulky and cannot be efficiently integrated into the autofocusing eyewear. See Ref. [2]. In addition, the eyewear needs only the parallax data, which is the angle between the two gaze directions. The autofocusing eyewear does not need the data regarding the absolute directions of the gaze of each eye, only the angle between the two directions.
What is needed is autofocusing eyewear that is compact and that has a real time parallax measuring subsystem. This subsystem must operate without the use of cumbersome optical attachments that restrict a user's field of view. Also, the subsystem must be lightweight. In this application a new type of autofocus eyewear is revealed and a practical eyetracking subsystem is taught that can meet all the above requirements. This is accomplished through the use of an eyewear integrated holographic infrared light illuminator and a Purkinje points imaging system, neither of which is perceived by the user.